build fails when _ANSI_SOURCE used

[goatshriek]

Describe the bug
Stumpless cannot be built when some symbols are missing, such as gmtime_r.

To Reproduce
The following configuration command can reproduce the errors initially reported:

cmake -E env CFLAGS="-D_ANSI_SOURCE -pedantic -Wall -std=c99" cmake ../stumpless

Expected behavior
At a minimum, the build should succeed in this configuration. Ideally, tests should also be able to run, but this is not absolutely required.

Additional context
Originally reported in the Stumpless community Gitter chat by @mk722

[goatshriek]

Progress on this issue can be tracked in the ansi-only-build branch.